An ion implanter includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam typically is mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and implanted into a target. The ion beam may be distributed over the target area by electrostatic or magnetic beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension.
Implantation of an ion species may allow a substrate to be cleaved. The species form microbubbles in the substrate material. These microbubbles are pockets of a gas or regions of an implanted species below the surface of the substrate that may be arranged to form a weakened layer or porous layer in the substrate. A later process, such as heat, fluid, chemical, or mechanical force, is used to separate the substrate into two layers along the weakened layer or porous layer.
Ostwald ripening may occur in substrates that have microbubbles. Ostwald ripening is a thermodynamic process where larger particles grow by drawing material from smaller particles because larger particles are more stable than smaller particles. Any atoms or molecules on the outside of a particle, which may be, for example, a microbubble, are energetically less stable than the more ordered atoms or molecules in the interior of a particle. This is partly because any atom or molecule on the surface of a particle is not bonded to the maximum possible number of neighboring atoms or molecules, and, therefore, is at a higher energy state than those atoms or molecules in the interior. The unsatisfied bonds of these surface atoms or molecules give rise to surface energy. A large particle, with a greater volume-to-surface ratio, will have a lower surface energy. To lower surface energy, atoms or molecules on the surface of smaller, less stable particles will diffuse and add to the surface of the larger, more stable particles. The shrinking of smaller particles will minimize total surface area and, therefore, surface energy. Thus, smaller particles continue to shrink and larger molecules continue to grow.
FIG. 1 is a view of Ostwald ripening in a substrate where microbubble size is not controlled. FIG. 1 is merely an illustration and is not to scale, although the x1 and x2 references will allow comparison of FIGS. 1-2 and 8-9. A species that forms the microbubbles 100 in the substrate 138 makes smaller microbubbles 101 and larger microbubbles 102. Due to their greater volume-to-surface ratio and lower surface energy, the larger microbubbles 102 will be more stable than the smaller microbubbles 101. To lower their surface energy, the smaller microbubbles 101 will diffuse to the larger microbubbles 102. Overall, the smaller microbubbles 101 may shrink and the larger microbubbles 102 may grow. Some of the species in the microbubbles 100 also may diffuse out of the substrate 138. Ostwald ripening and diffusion of the species out of the substrate 138 will affect the substrate 138 when it is cleaved along the weakened layer or porous layer represented by line 103.
Experiments with hydrogen implantation at a temperature between 40° C. and 100° C. suggest that at least some of the microbubbles 100 increase in size above approximately 50° C. or 60° C. Consequently, surface roughness may be increased if the temperature of substrate 138 is raised during implantation.
FIG. 2 is a view of the substrate of FIG. 1 after the substrate is cleaved. The substrate 138 in FIG. 1 was cleaved along the weakened layer or porous layer represented by line 103. As illustrated in FIG. 2, significant surface roughness 104 occurs due to Ostwald ripening and diffusion of the species out of the substrate 138. Due to the rough surface within a substrate when the substrate is separated into two layers along the weakened layer or porous layer, a polishing step after the substrate is cleaved may be required to make the surface of the substrate smooth enough for device manufacture. This polishing step is expensive and compromises the uniformity of the silicon on the surface of the substrate. Accordingly, there is a need in the art for an improved implantation process and, more particularly, an implantation process that will reduce the size at which microbubbles are stable within a substrate.